I speak from the Hadiths (the sayings of Muhammad), the biographies of the Companions and the knowledge of the saints in Islam.
Muhammad (peace be upon him and his family) was succeeded by Abu Bakr (may God be well pleased with him) on the throne of the kingdom of Islam. Abu Bakr was succeded by Umar (may God be well pleased with him). During his lifetime, Muhammmad (peace be upon him and his family) would always say I, Abu Bakr and Umar did this, I Abu Bakr and Umar went there, etc...
Umar was the 40th person to accpet Islam. Before he embraced Islam, the Muslims were hiding. When he embraced Islam, he advised Muhammad to go out along with the Muslims. So the Muslims came out, under the leadership of Hamza (the uncle of Muhammad) and Umar.
He is the son in law of Muhammad, who married his daughter Hafsa, the Mother of the Believers.
Umar made Islam to expand greatly during his 10 year rule. He conquered among others, Jerusalem and Persia (Iran). Jerusalem remained Muslim until the 20th century.
God mentions Abu Bakr and Umar in this verse:
"and consult them in important affairs" (Surah 3, V 159)
Umar told Muhammad, I love you more than anything except myself. Muhammad replied, you must love me more than yourself. Umar said, now I love you more than myself. Muhammad approved of it.